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This isn’t even a question of training data, thy fed the full git source code directly to the llm.

I would say it's worse, the whole C Git source code is checked in https://github.com/gitbutlerapp/grit/tree/main/git

I wonder if imitating clean room reverse engineering with two LLMs would be enough for licence compliance.

That already exists[1]. It looks like a joke but apparently they will accept your money to do it, which seems to cross the line of a joke.